Los Angeles, California - A dangerous heatwave was building over parts of the western US Tuesday, with forecasters warning of rocketing temperatures in an early taste of a possibly brutal summer for the region.

The mercury was expected to top out at well over 100 degrees Fahrenheit, with some areas experiencing highs as much as 30 degrees above normal for this time of year.

Southwestern desert areas and California's Central Valley fruit basket were set to be particularly unpleasant, the National Weather Service warned.

"Widespread temperature records are expected to be tied or broken across much of the aforementioned areas," the agency said.

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Temperatures in Death Valley were set to hit a deeply uncomfortable 122F on Thursday, while Las Vegas could be baking in 112F heat.

Forecasters issued excessive heat warnings for parts of Nevada, Arizona and California, with the heat expected to spread further inland as the week goes on.

"Little overnight relief will make for dangerous conditions for those without effective cooling and/or adequate hydration," the NWS said.

California's Environmental Protection Agency said certain groups were particularly at risk.

"Extreme heat is an invisible but dangerous consequence of climate change, and California's outdoor workers, seniors, and children are particularly vulnerable," it wrote on social media.

Southern Texas was also experiencing sweltering temperatures, with the border city of Rio Grande expected to see highs of 117F later Tuesday.

Forecasters in the US are watching for the development of a ridge of high pressure that would bring more heat in from Mexico, which has been withering under a punishing heatwave.

Late last month, Mexico City – which sits 7,350 feet above sea level and has traditionally enjoyed a temperate climate – logged its highest-ever temperatures.

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Officials say dozens of people have died in repeated heatwaves that have scorched the country, with hundreds of others sickened.

Experts say there could be worse to come.

This year is on course to be "the warmest year in history," Francisco Estrada, coordinator of the Climate Change Research Program at the National Autonomous University of Mexico, has warned.